Stabilized ethylene-tetrafluoroethylene copolymer composition

ABSTRACT

Ethylene-tetrafluoroethylene copolymer compositions are stabilized against thermal degradation so as to render them meltprocessable by imcorporating therein 0.001 to 5 weight percent of Alpha -alumina, based on the weight of copolymer in the compositions.

llkihashi et a1.

STABILIZED ETHYLENE-TETRAFLUOROETHYlLENE COPOLYMER COMPOSITION Inventors: Hiroshi Ukihashi; Masaaki Yamahe, both of Tokyo; Hiromichi Nishimura; Haruhisa Miyake, both of Yokohama, all of Japan Assignee: Asahi Glass Company, Ltd., Tokyo,

Japan Filed: Dec. 26, 1973 Appl. NO: 428,414

Foreign Application Priority Data Field 0fSearcl1..... 260/4227, 45.7 R, 45.75 R, 260/875 B [56] References Cited UNITED STATES PATENTS 2,468,664 4/1949 Hanford et al 260/875 2,737,505 3/1956 Finholt 260/4575 3,060,098 10/1962 Gcrshon 51/309 3,121,623 2/1964 Nesin 51/293 3,190,864 6/1965 Shipp et a1 260/921 Primary Examinerl\/1elvyn I. Marquis Attorney, Agent, or FirmOblon, Fisher, Spivak, McClelland & Maier [57] ABSTRACT Ethylene-tetrafluoroethylene copolymer compositions are stabilized against thermal degradation so as to render them melt-processable by imcorporating therein 0.001 to 5 weight percent of a-alumina, based on the weight of copolymer in the compositions.

5 Claims, No Drawings STABILIZED ETHYLENE-TETRAFLUOROETHYLENE v COPOLYMER COMPOSITION BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a stabilized ethylene-tetrafluoroethylene copolymer composition which is meltprocessable, and more particularly, this invention relates to an ethylene-tetrafluoroethylene copolymer which is stabilized against degradation during fabrication and use at elevated temperatures. This invention also relates to a process for preparing the stabilized compositions.

2. Description of the Prior Art It is known that ethylene-tetrafluoroethylene copolymers have excellent thermal, chemical, electrical and mechanical properties and are melt-processable. These copolymers are known to be heat resistant resins. However, under atmospheric conditions and at temperatures near their decomposition points, e.g. at higher than 310C, the conventional ethylene-tetrafiuoroethylene copolymers become slightly colored after about 1 hour. Not only do they become reddish brown in color but the coloring is accompanied by foaming.

The undesirable coloration and foaming of the conventional ethylenetetrafluoroethylene copolymers is a particularly significant disadvantage since many applications of these copolymers are at sufficiently high temperatures to cause such thermal decomposition. Also, when a molding apparatus is used to form molded articles of these copolymers, the molding temperatures may exceed 310 C for a sufficient time to cause thermal decomposition which disrupts the molding process.

A need therefore exist for a thermally stable ethylene-tetrafluoroethylene copolymer which is meltprocessable and does not suffer from the above described disadvantages.

SUMMARY OF THE INVENTION Accordingly, one object of this invention is to provide a process for improving the thermal stability of an ethylene-tetrafluoroethylene copolymer.

Another object of this invention is to provide a meltprocessable composition comprising an ethylene-tetrafluoroethylene copolymer and a stabilizer therefore.

Yet another object of this invention is to provide a melt-processable composition comprising an ethylene-tetrafluoroethylene which is resistant to thermal degradation on prolong exposure at an elevated temperature.

Briefly, these and other objects of the invention as hereinafter will become apparent are achieved by intimately dispersing a small amount of a-alumina in the ethylene-tetrafluoroethylene copolymer.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS.

The composition of this invention has a high thermal stability without coloring or foaming upon exposure of the composition to a temperature of higher than about 310C under atmospheric conditions for about 3 hours.

Moreover, the addition of a small amount of a-alumina to the copolymer does not adversely affect the physical and electrical properties and melt-processability of these copolymers.

In this invention, it is important that a-alumina is combined with the copolymer. Other metal oxides such as 'y-alumina, titanium oxide, magnesium oxide, zinc oxide, tin oxide, calcium oxide, or the like impart a slight thermal stabilization effect against decomposition of the copolymer, and in some cases, the metal oxide reduces the thermal stability of the copolymer.

The type of a-alumina used in the invention is not particularly limited, however, it is preferable to optimize the particle size, the specific surface area, particle distribution or the like of the a-alumina depending on the desired properties of the copolymer composition. For example, it is preferable to use an (Lt-alumina having a relatively small particle diameter which is usually less than 100,8 and preferably about 1 505. It is also preferable to have a sharp particle distribution. The specific surface area of the oz-alumina is from about 0.1 3O m lg and preferably from about 0.5 5m /g. When the particle diameter and specific surface area of the a-alumina is not included within the above ranges, disadvantages result which decrease the improvement in the thermal stability, the surface smoothness of a processed article made of the composition, and the uniform compatibility of the oz-alumina and the ethylene-tetrafluoroethylene copolymer a-Alumina is preferably prepared by calcination at a temperature higher than 900C and preferably higher than 1000C. I

The amount of a-alumina added is usually from about 0.001 5 wt.% and preferably from 0.005 3 wt.% based on the weight of ethylene-tetrafluoroethylene copolymer. When the amount of a-alumina is too small, the improvement in thermal stability does not result and when the amount of a-alumina is too large, the mold processability, that is, the fluidity of the ethylene-tetrafluoroethylene copolymer is decreased and mold processed article is cloudy and does not possess the excellent properties of the copolymer.

The ethylene-tetrafluoroethylene copolymers used in the invention can be prepared by various well known polymerization methods such as a catalytic emulsion polymerization in an aqueous medium, a suspension polymerization; catalytic solution polymerization; vapor polymerization; radioactive irradiation polymerization, or the like.

The ratio of ethylene to tetrafluoroethylene in the copolymer may vary over Wide limits. For example, the molar ratio of tetrafluoroethylene to ethylene may be from 40/60 to /30 preferably from about 45/55 to 60/40. It is preferable to use copolymers having a volumetric melt flow rate of 10 300 mm /sec., preferably from 20-160 mm sec. The volumetric melt flow rate used in the specification is defined as follows: 1 g of a sample of the copolymer is extruded through a nozzle having a diameter of 1 mm and a round of 2 mm under a predetermined pressure of 30 kg/cm at a predetermined temperature by using a Flow tester. The volume of the molten sample extruded per unit time is shown as the volumetric melt flow rate and the unit is mm /sec. The predetermined temperature can be the temperature range at which the ethylene-tetrafluoroethylene copolymer is moldable. That is, a temperature range between the temperature at which melt flow begins and the temperature at which thermal decomposition begins and it is near the temperature at which melt flow begins, i.e. the fluid initiation temperature.

3 The predetermined temperature is selected from 260 360C for ethylene-tetrafluoroethylene copolymer. The temperature used to measure the volumetric melt flow rate is lower than the temperature at which thermal decompositionof the copolymer begins.

Small amounts of a cornonomer such as propylene, isobutylene, vinylfluoride, hexafluoropropylene, chlorotrifluoroethylene, acrylic acid, acrylic acid alkyl esters, chloroethyl vinyl eter, perfluorovinyl ether, or the like and a modifier may be added to the tetrafluoroethylene-ethylene copolymer. i i 4 The copolymers of this invention have excellent heat resistance, chemical resistance, electrical properties, mechanical properties and also mold processability under heat-melting. v

It is also possible to add suitable reinforcing agents, fillers, lubricants, pigments, or the like to the copolymer composition of the invention. The addition of the additives can improve the surface hardness; mechanical strength, abrasion resistance and other properties of the copolymer composition. I

The method" of addition and mixing of the a-alumina is not. particularly limited, however, it is preferable to use a methodwhich uniformly mixes the ethylene tetrafluoroethylene with then-alumina. For example, a-alurnina may be crushed into particles having a particle diameter and particle distribution with the aforementioned ranges and mixed with the ethylene-tetrafluoroethylene copolymer or ethylene-tetrafluoroethylene copolymer may be crushed and mixedor'it'may be used in a molten state. it is also possible to disperse the ethylene-tetrafluoroethylene copolymer in an aqueous medium or an organic medium and then to mix it with a-alumina by a wetmixing method. I i i The composition. of, the invention can be, used for preparing various types of molded articles by various processing methods, and can be applied to various applications in various uses. For example, the copolymer composition may be'used for press molding, extrusion' molding, injection'molding,a lining, a'coating, a

wire coating, or the like.

The optimum ethylene-tetrafluoroethylene copolymers have a temperature at which melt flow begins of about 260 300C and a temperature at which thermal decomposition begin of about 320 360C. The melting temperature of the copolymer is selected to be between the temperature at which melt flow begins and to heat the composition at higher temperaturethan. that of the. copolymer having no a-alumina whereby the processa bility of the mold processing is greatly improved.

Having generally described this invention a further understanding can be obtained ,by-reference to certain specific examples, which are included for purposes of illustration only and are notzintended to be limiting unless otherwise specified.

. The resultsof the served.

EXAMPLE 1 parts of wt. of a ethylene-tetrafluoroethylene copolymer (having a molar ratio of tetrafluoroethylene to ethylene of 52 to 48,.a fluid initiation temperature of 279C, a temperature at which thermal decomposition begins of 345C and a volumetric melt flow rate of 46 mm /sec. at 300C) was admixedwith 0.1 part by wt. of a-alumina having anaverage particle. diameters of 5p; The resulting composition was press-molded at 300C to form a sheet. The sheet ,was exposedto the atmosphere at 3 10C- for 3 hours. No coloring or foaming 'was observedfOn the other hand, when a-alumina was the atmosphere at 3lOC for 2.5 hours. No coloring or foaming was observed.

' EXAMPLE 3 AND REFERENCES 1 4 The ethylene-tetrafluoroethylene copolymer of EX- AMPLE l was admixed with 0.1 wt.% of each of d-alumina (average particle diameter of 16p. a-alumina, zinc oxide, magnesium oxide, or calcium oxide to prepare each composition.

I; The composition was pressedat 300C to" prepare sheet. ,1 K I I al stability tests on the sheets are shown in Table l. Coloring and foaming were observed whenthe metal oxides other than a-alumina were used.

TABLE-I i Appearance after heating Additive at 310C for 2 hours Example 3 a-AlgO, no change Ref. 1 'y-Al o 1 surface roughness and black brown color Ref. 2 ZnO black gray color Ref. 3 MgO reddish brown color surface foams Ref. 4 CaO reddish brown color surfacefoams EXAMPLE 4 I EXAMPLES 5 5 7 I The ethylene-tetrafluoroethylene copolymer of EX- AMPLE 4 was admixed with varying amounts of B-alumina having an average particle diameter of 2p. by kneading the mixture of a-alumina and copolymer. The

resulting compositions was press-molded at 300C to form a sheet. The sheet was exposed to the atmosphere at 310C for 2.5 hours. The amount of oz-alumina added and the appearance of the sheets are shown in Table 11.

EXAMPLE 8 An ethylene-tetrafluoroethylene copolymer (having a molar ratio of tetrafluoroethylene to ethylene of 53 to 47 and a small amount of hexafluoropropylene) (volumetric melt flow rate of 52 mm /sec. at 300C) was admixed and kneaded with 0.05 wt.% of oz-alumina having an average particle diameter of 1,u.

The resulting composition was press-molded at 300C to form a sheet, which has a tensile strength of 540 kg/cm and an elongation of 400%. The sheet was exposed to the atmosphere at 320C for 1.5 hours. No coloring or foaming was observed. p

On the other hand, when a-alumina was not added, the sheet has a tensile strengthof 450 kg/cm and an elongation of 420% and the sheet was colored byexposing to the atmosphere at 320C and was changed to reddish brown with foam after 1.5 hours.

EXAMPLE 9 clearly indicating the thermal degradation of the sheet.

EXAMPLE 10 An ethylene-tetrafluoroethylene copolymer (having a molar ratio of tetrafluoroethylene to ethylene of 53 to 47 and a small amount of hexafluoropropylene)(- volumetric melt flow rate of 25 and mm /sec. at 300C) was admixed by kneading with 0.05 wt.% of ot-alumina having average particle diameter of 2 The resulting composition .was press-molded at 300C to form a sheet. The sheet was exposed to the atmosphere at 310C for 2 hours. No coloring or foaming was observed.

Having now fully described the invention, it will be apparent to one of ordinary skill in the art that many changes and modifications can be made thereto without departing from the spirit or scope of the invention as set forth herein.

What is claimed as new and intended to be covered by Letters Patent is:

11. An ethylene-tetrafluoroethylene copolymer composition having improved thermal stability, which comprises:

an admixture of an ethylene-tetrafluoroethylene copolymer, wherein the mold ratio of tetrafluoroethylene to ethylene is 40 70/60 30 and from 0.001 to 5 wt. of oz-alumina, based on the weight of the copolymer.

2. The composition of claim 11, wherein the copolymer has a volumetric melt flow rate of about 10 300 mm /sec. I

3. The composition of claim 1, wherein the copolymer has a flow initiation temperature of 260 300 C and a thermal decomposition temperature of 320 360C.

4. The composition of claim 1, wherein the ot-alumina has an average particle diameter of less than a and a specific surface area from 0.5 5 /g.

5. The composition of claim 11, wherein the a-alumina is calcined at a temperature higher than about 900C. 

1. AN ETHYLENE-TETRAFLUOROETHYLENE COPOLYMER COMPOSITION HAVING IMPROVED THERMAL STABILITY, WHICH COMPRISES: AN ADMIXTURE OF AN ETHYLENE-TETRAFLUOROETHYLENE COPOLYMER, WHEREIN THE MOLD RATIO OF TETRAFLUOROETHYLENE TO ETHYLENE IS 40-70/60-30 AND FROM 0.001 TO 5 WT. % OF A-ALUMINA, BASED ON THE WEIGHT OF THE COPOLYMER.
 2. The composition of claim 1, wherein the copolymer has a volumetric melt flow rate of about 10 - 300 mm3/sec.
 3. The composition of claim 1, wherein the copolymer has a flow initiation temperature of 260* -300 *C and a thermal decomposition temperature of 320* - 360*C.
 4. The composition of claim 1, wherein the Alpha -alumina has an average particle diameter of less than 100 Mu and a specific surface area from 0.5 - 5 2/g.
 5. The composition of claim 1, wherein the Alpha -alumina is calcined at a temperature higher than about 900*C. 